Wave transmission system



Aug 18, 1931. R. c.. MATHEs 1,819,648'

WAVE TRANSMISSION SYS'TEM I Filed March 30, 1929 5 Sheets-Sheet l vvl/5v TOP R. C MAT/15s ATm/P/VEY,

Aug. 18, 1931. l R1 MA1-HES. 1 1,819,648 l WAVE TRANSMISSION SYSTEM Filed March 30, 1929 5 Sheets-Sheet 2 Aug. 1-8, 1931. R. c. MATHEs WAVE TRANSMISSION SYSTEM Filed March so. 192s 54 sheets-sheet 3 /NVENTOR' R. CMA THe-.s

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ATTORNEY R. c. MATHEs 1,819,648

WVE TRANSMISSION SYSTEM Filed March 50, 1929 5 Sheets-Sheet 4 Aug. 18, 19311.

Aug.r 18, 1931.'`

- R. c:. -MArI-nss WAVE TRANSMISSION SYSTEM 5 sheets-sheet 5 Filed Maron so, 1929 /NvENToR R. C'. MA THES Arm/ver f)- privacy apparatus.

Patented Aug. 1s,v 1931 UNITED STATES PATENT OFFICE ROBERT C. MATHES, OF WYOMING, NEW JERSEY, ASSIG-NOR TO BELL TELEPHONE LABO- RA'l'ORIIIIiIS, INCORPORATED, 0F NECW YORK, N. Y., A CORPORATION AOlli NEW YORK WAVE TBANsmIssIoN SYSTEM Appncatin med March ao, 1929.V serial no. 351,415.

This invention relates to wave transmission and' especially tio-privacy systems of com-- munication.

,An obJect of thev invention 1s economy of lAnother object is to obtain greater privacy for signals transmittedin a given direction in a two-way signaling system than is obtained by merely the'alteration of those signals which occurs before they are sent 'om their transmitting station.

In onerspecific form of the invention pin vwhich privacy in radio -telephony results from relatively delaying frequency bands of speech energy in the ment of those bands for the other direction.

This dissimilarit-y in the kinds of alteration introduced in sending signals from the two 'intercommunicating stations to reduce their intelligibility as sent out, requires that a' listener-in have two separate receiving systems, and if the same frequencies be tra-nsmitted in both directions, each of the receiving systems will need vto have a sharp directional selectivity, a requirement very diicult to meet except in limited areas in relation to the twoy transmitting stations.

Othe objects and features of the invention will be apparent from the following -description and claims.-

Fi' s. 1 and 2 show the two terminals, respectlvely. of a four-wire type of circuit embodying one form'of the invention;

Fig. 3 shows one terminal of a two-Wire type of circuit embodying a second'form of the invention; 1

^ Fig. 4 shows a terminal adapted for either a four-wire Yor two-wire type of circuit embodying the invention in still different form; and Fig. 5 shows-an acoustic delay means suitable for use in the system of Fig'.j 4v and also in the systems of Figs. 1 to .=3.

To make it more ditlicult for an outsider to general manner dis-z closed in Mathes Patent 1,542,566, June 16, 1925, the same' delay apparatus serves for- -both directions oftransmission in a two-way listen in on a two-Way radio or other signal- Y ing system with provision for secrecy, there may be used dissimilar ways of rendering the transmission in the respective directions unintelligible. That is, privacy may be augmented by renderng thesignals sent in one direction from one terminal unintelligible, in

A011e manner, and rendering the signals sent in the opposite direction from the other terminal unintelligible, in another manner. The one manner may be by alteration of one characteristic of the signaling waves (as for example by use of frequency ban inversion or transportation, or both, for the onedirecf tion of transmission), and the other manner may be by alteration of another characteristic (as for example by use ofV frequency band delay for the other direction); or the one manner may be by alteration of one characteristic in a given way and the other manner by alteration of the one characteristic in a diiierent way. Three examples of this latter type of system are as follows: F lrst, in the case of secrecy by frequency band inversion (as vdisclosed for-instance by Mathes Patent 1,542,565, June 16, 1925), there may be dissimilarity, for the two directions of trans-- mission, as to which bands are left normal and which are inverted. Second, in the case of secrecy by frequency band transposition (as disclosed for instance in Espenschied Patent 1,546,439, July 21, 1925) there may be dissimilarity, for the twofdirections of transmission, as to the specific transpositions made. Third, in the case of secrecy by frequency band delay (as disclosed for instance 1n Mathes Patent 1,542,566, mentioned above),

'there may be delay dissimilarity for opposite directions.v This delay dissimilarity, is referred to above, and examples of it are described hereinafter.

In order that advantage may betaken of the optimum setting for the band edges in those secrecy schemes which involve splitting.

the speech energy into bands,'vario us dissimilarities of operation for the opposite directions of transmission vmay be introduced 'while using the same band separation in the two directions.` In the case of frequency band inversion some dissimilarityl may be introduced in the selectionof which bands are left normal and which bands are inverted. A similar choice is possible in the case of frequency band transposition. f

In the case of frequency band delay, dissimilarity may be introduced for example by selecting different bands to which are given ythe maximum and minimum delay on transmission. The following table shows now, if (as in the system of Figs. 1 and 2 about to be described) the speech energy be divided'into three frequency bands and the higher -bands be delayed most on transmission from one end, which for convenience may be called the west end, while the lower bands are delayed most on transmission from the other or east end,

the amount of confusion caused to an outside listener adjusted for listening to west to east transmission would be increased for the transmission from east to west.

Outside lis- 1 l tener adjusted Transmission west to east for west to east transmission s d Receiv- R sending dglamg mg 9" Total eclv- Total yat rection ing corband Westend at east delay rection delay end 0F| O 2T 2T 2T 2T VFi--Fz T T 2T T 2T Fz-Fa 2T O 2T 0 2T Transmission east to west l S d Receivsending e 1g mgof'frotal delay at rection band eastend at west delay end o-ra A 2T o 2T 2T 4T Fx-Fz T T 2T T 2T Fz-Fs O 2T 2T O O A' listener-in would therefore need two separate receiving systems so that the transmission froin each terminal could be individ-` ually picked up and corrected for; and in, for example, a radio telephone system sending'the same frequency in both directions, each of those two receiving systems would need to have sharp directional selectivity, which, as noted above, is a requirement very difficult to meet except in limited areas in relation to the two transmitting stations.

The syste-n1 of Figs. 1 and 2 may be, for example, a radio telephone system of the type referred to in connection with the table above. For convenience, the terminal shown in Fig. 1 will be designated the west terminal of the system and the terminal shown in Fig. 2 will between lines 5 and 6. For example, line 3 ,may connect with line 4, and line 5 may connect with line 6. Or, on the other hand, line 3 may terminate, at its east end, in a radio transmitter, and l-ine 4 may have at its west end a radio receiver for receiving from that transmitter; and line 5 may terminate at its west end in a radio transmitter and line 6 may have at its east end a radio receiver for receiving from that transmitter. In the lcase y of s iich a radio system, the carrier frequency employed for the two directions of transmission may be the same for both. directions, in order that thesignaling waves passing in opposite directions may be of the same frequency range for both directions, so that a listener-in will need directionally selective receiving systems as noted above.

The frequencies transmitted in lines 3, 4, 5

4and 6 may be the same as those transmitted in.

lines 1 and 2. p

This system illustrates the Ease in which speech is divided into three frequency bands that are sent at different relative times';

AIn this case two delay circuits on paths 10 and 11 are used for' giving the three relatively different sending times, each delay introducing a delay of time T. The filters used for dividing the speechinto frequency bands have their pass ranges indicated on the drawing. Starting from line 1 at the west terminal the low frequency band O Fl, passes through a low pass filter 12 onto line 3 of the secrecy part of the system, with no delay. The mid-frequency band, ffl-F2, passes through a band pass filter 13, the delay circuit or path 11, an amplifier A'-14, and a high pass lter 15 onto line 3, and so is given a delay of time T relative to the low frequency band in getting to the secrecy part of the circuit. The high frequency band, Fz-oo A' A-14 and filter 15, onto line 3, and so is given a delay of 2T relative to the low frequency band.

At the other end of the system in passing' from the secrecy part of the circuit of the two-wire line 2 at the east terminal, the high frequency band passes through a high pass filter 19, but no delay path; the mid-frequency band passes through a low pass filter 20, a delay circuit or path 21 which introduces a delay of time T, an amplifier A-23, and a band pass filter 2,4 onto line 2; and the low frequency band passes through .elements 20,A

21, A`23, a low pass filter 25, a delay circuit or path 22 which introduces adelay of time T, an amplifier A-26 and a low pass filter 27 onto line 2. Thus, the high frequency band is given no additional delay, the mid-frequency band is given a delay of T, and the low frequency band is given a delay of 2T, so that all three bands have been given the' same overall delay of 2T and are back in normal time relation with respect to one another.

In .the case of transmission from east to West the same ,-delays are introduced in reversed order with respect to the three bands y at the transmitting and receiving ends.

Starting from line 2 at the east terminal the high frequency band, FZ-oo passes through a high pass filter 29 onto line 5 of the` secrecy part of the system with nodelay. The mid-frequency band, lFl-FZ, passes through a band pass filter 30,a delay circuit or path 32 of delay time T, an amplifier A-33, and a low pass filter 34 onto line, and so is given a delay of time T relative -to the high frequency band in getting to the secrecy part of the circuit. The low frequency band, 0-F1, passes through a low pass filter 35, a delay circuit orpath 31 of delay time T, an amplifier 1 -,36, a low pass filter 37, the delay path 32, amplifier A-33 and filter 34 onto line 5, and so is giveri a delay of 2T relative to the high frequency band.

At the other or West end of the system, in passlng froml the secrecy part 6 of the circuit to the two-wire line 1, the low frequency band passes through a low pass filter 38, but no AIdelay path; the mid-frequency band passes through a high pass filter 39, a delay circuit 'or path 40 of delay time T, an amplifier A-42, and a band pass filter 43 onto line 1;

and the high frequency band passes through elements 3 9, 40, .4;42, a high pass filter 44, a delay circuit or path 41 of delay time T, an amplifier .lf-45 and a high pass filter .46 onto line 1. Thus, the lowfrequency band is given yno additional delay, the mid-frequency band 1s given a delay of T, and the high frequency band- 1s given a delay of 2T, so that all three bands have been given the same overall delay of 2T and are back in normal time relation With respect to one another.

The gains of amplifiers A-l4 and A-17 may be made such that all of the frequency 4 'bands suffer equal transmission losses in passing from'line l to line 3. The gains of amplifiers A-23, Ak26, A*33, A-36, A-42 and A45may be adjusted for a similar purpose.

It is sometimes desirable to have the secrecy apparatus in the two-wire portion of the transmission circuit, instead of in the four- Wire portion. Fig. 3 showsone terminal, for convenience say the west terminal, of a twowire type of secrecy system in general similar to the four-Wire type of secrecy system shown in Figs. 1 and 2. This terminal circuit shown Iuse of dashes to represent this line indicates that it may be of considerable extent and/or l that any desired apparatus, as for example,

the radio transmitters and receivers mentioned 1n connection with the system of Flgs.

1 and 2 may be included in the link between the two terminals of the system. For such Ainclusion of a radio link, the usual manner offconnecting a two-Wire line such as 1' to a four-wlre type of circuit including a radio vline can be employed, (and an illustration is found in the case of the connection of line 1 to lines 3 and 6 in Fig. 1 by means of bridge transformer or hybrid coil BT and line balancing network N, considering the secerecy apparatus to be omitted from that figure).

In the system of Fig. 3, as in that of Figs.

1 and 2, the speech is divided into three 'frequency bands that are sent at different relative times. The filters used for dividing the speech into frequency bands have their pass ranges indicated on the drawings.v Starting from line l at the west terminal the low frequency band, 0-F1, passes through bridge transformer BT, conductors 50, conductors 5l, a law pass filter52, loss equalizer 53, loW pass filter 54, conductors 55, amplifier A-56, and bridge transformer BT onto line 1' with no delay. The mid-frequency band, F1-F2, passes through elements BT, 50, 51, a band pass filter 62, loss equalizer 63, band pass filter 64, delay circuit or path 65 of delay time T, high pass filter 66, and elements 55, A-56, and BT, onto line 1', and so is given a delay of time T relative to the low frequency band in getting to the secrecy part 1" of the circuit. The high frequency band, Fg-oo,

-passesfthrough elements BT, 50, 51, a high pass filter 72, delay circuit or path 73 of delay time T, high pass filter 7 4, and elements 65, 66, 55, A-56,'and BT, onto line l and so is given a delay of 2T relative to thel low frequency Iband.

The east terminal (not showp) the' system of Figs. 1 and 2, the low frequency band, instead of the high frequency band,"

is like the- -West terminal except that, as in the case of merely converting the four-wire portion of the circuit at each terminal into a two-wire circuit by means of a hybrid coil in a mani path 87'of delay time T, akhigh pass filter" 88, and elements 84, 85, 79, A-S() and BT Thus, the low frequency band two control amplifier rectifiers Alt-91 and' -A;-80and BT onto line 1.

line 1, the low frequency band passes through bridge transformer BT,-conductors 75, low pass filter 76, transmission loss equalizer 77, low pass filter 78, conductors 79, amplifier A-80 and bridge transformer BT onto line 1. The mid-frequency band passes through elements BT, 75, a band pass filter 81, transmission loss equalizer 82, band pass filter 83, delay circuit or path 84 of delay A* time T, high pass filter 85, and elements 79,

The high frequency band p asses through elementsBT, 75, a high pass filter 86, a delay circuit or onto line 1. A is given nok additional'delay the mid-frequencybandis given a delay of T, and the high frequency band is given a delay of 2T, so that all three bands have been given the same overall delay of 2T and are back in normal time relation with respect tovone another.

The transmission loss equalizers 53 and 63 may be made .to have lsuch loss that all of the frequency bands suffer equal l.transmission loss in passing from line 1 to amplifier A-56. The loss o f equalizers 77 and V82 maybe adjusted to give zero transmission loss, or a desired gain, from line 1 to line 1 and vice versa.

In connection with the circuit shown in Fig. 3 two-wire echo suppressor arrangements may be applied. For example, the

AR-92 may have ytheir input sides connected tothe bridge points of the hybrid coils through contacts of relays 93 and 94, these contacts being included in conductors .50 and 75 respectively.an d these relays being operated by the amplifier rectifier ARF-92 and .AR-91 respectively. By also having .AR- 91 open the input circuit of A--SO at contacts of relay '95 which .are included in the conductors 79 and having .AR-92 open the input circuit of A-56 at contacts of relay 96-which are included inthe conductors Y55, the suppressor `circuit will have characteristics part way between those offa two-,wireksuppijessqr circuit nbt lprovided with delay circuits and a two-wire suppressor circuit with delay circuits as shown for example in Mathes Patent 1,698,955, December 4, 1928, or a four-wire echo suppressor circuit of the type shown for example in Fig. 1 of E. D. Johnsonk Patent 1,526,550, February 17, 1925, for two bands of the speech energy in the case of .transmission in each direction are delayed between the two hybrid coils, corresponding to the time delay in the two halvesofa four-Wire circuit. Thus, reliability of operation isincreased and the time during which circulating cur- -rents can build up is reduced.

Although in both of the systems described above, the dissimilarity of signal distortion for opposite transmission directions is dissimilarity with respectkto relative time displacement of frequency bands, other kinds of dissimilarity may be employed, as indicated i above. For example, the one-waysystem shown in Mathes `Patent 1,542,565, June 16, 1925, in which secrecy is obtained by inversion of frequency bands, may be used with an oppositely ,directed one-way system like that of Mathes kPatent 1,542,566 referred to above, in which secrecy is obtained by relative time displacement ofthoselfrequency bands; ortransmission in one direction may be in accordance with Espenschied Patent 1,546,439, July 21, 1925, in which secrecy is obtained"vv by frequency band transposition and by frequencyk band inversion and transposition, and transmission in the opposite direction may be with the system ofthe Mathes Patent 1,542,566, so that secrecy will be 'augmented by having delay ofI frequency bands in transmission in one direction and either transposition or inversion, or both transposition and inversion, of those bands in transmission in the other direction; or 1n case secrecy is obtained in one direction of transl mission by inversion of frequency bands as in Mathes Patent 1,542,565, for transmission in the opposite direction, Y the system can be duplicated except as to the particular ones of those frequency bandsthat are left-normal and the particular ones that are invented; or in the two-way system `of the Espenscliied Patent 1,546,439, in which secrecy is obtained by transpo'ing frequency bands, dissimilarity can be ntroduced, for the two directions of transmission, as to the specific transposi- .tion of those bands that is made. Moreover,

where the dissimilarity for transmission in opposite directions is dissimilarity with respect vto relative time displacement of frequency bands, the dissimilarity need not vbe obtained by assignment of delays changing' with frequency in one order for one direction of transmission and in the reverse'order for .the oppositev direction, as in the system ofv Figs.. 1 and 2 and the system of Fig. 3.A For example, the relative delays given to the bands may differ for opposite directions of cept that a different type of se recy apparatus is employed between the tw wireline 1 and the lines Sand 6 and correspondingly such different type of secrecy apparatus is emv ployed at the east terminal (notshown) between the two-wire line and the four-wire or secrecy vportion of the system. 1 s

In circuits where voice operated repeater switching is desired together with conversion to secrecy, asfor example, in the circuits at 'the two terminals of the present commercial transatlantic radio telephone system in which (as in the disclosure in' Nyquist Patent 1,607,687, November 23, 1926) the receiving apparatus is normally on and the transmitting apparatus is only connected in when out'- going speech comes along, delay circuitsV or paths may be used vfor conversion to secrecy and the same delay means may serve to assist in reducing'clipping or elimination of portions of the signals because of the time required to effect switching. A way in which lthe voice operated control circuits sand the delay circuit to avoid clipping on transmitting canv be introduced is shown in Fig. 4 and will appear as4 the description of that figure proceeds.

Systems in which some outgoing talk j passes through delay circuits in tandem at one terminal, such for example as the systems-in Figs. 1, 2 and' 3, provide for economical use of delay circuit material, which is of importance when electrical delaycircuits are used in which the cost goes up in proportion to the amount of delay. On the other hand, if acoustic or mechanical delay circuits or paths are used the distortion of the apparatus for converting from electrical t`o mechanical vibrations and back again piles up and may become too large, especially in cases in which bands of speech energy must pass through a number o f such conversions and back again at the two terminals of the circuit. f

The circuit of Fig. 4 avoids such cumulative distortions. In this type of circuit the total length of delay path is increased, the amount of conversion equipment is the same, while the number of conversions any one band of speech energy must pass through at both ends ofthe secrecy system is reduced to two.

In the case' of acoustic delay paths the increased cost of increased total length of path will probably be negligible.

To function with the minimum amount of equipment as shown, dissimilar transmission in the two directions may be used with the in;

creasing delays being assigned to opposite dierating ductors 100 to amplifier-rectifier Alt-101 which operates relay 102 for opening a short circuit through its` upper contact across conductors 103 and for opening the circuit through conductors 104 at its lowercontacts. The other portion passes through conductors 105 and then divides into three frequency The low frequency band passes through low pass yfilter A, delay path 106 of delay time filter A', amplifier A-108 and conductors 103 onto line 3. (The pass ranges of the filters shown -in Fig. 4 areindicated on the drawings. The delay paths shown in the figure maybe electricaldelay circuits or acoustic or other mechanical delay paths.) The mid-frequency band passes through band pass filter B, delay path 110 of4 delay time't-l- T, transmission loss equalizer 111, band pass filter B, amplifier Arf-108 and conductors 103 onto line 3. The high frequency band passes through( high pass vfilter C, delay path 112 of delay time t+2T, high pass filter C', amplifier 1-108, and conductors 103 onto line 3.

, Speech energy coming from line 6 for delivery to line 1 passes through theF lower con- .t, transmission loss equalizer 107, low pass.-

tacts of relay 102'and then divides into two put circuit of Alt- 101. The other portion divides into three frequency bands of ranges 0-F1, Fl-FZ, and F2-oo. The-high frequency `Band passes through high ass lter D', transmission loss equalizer 10 delay path 106, highl pass filter D, and amplifier A-1l8 which feeds into line 1 through bridge transformer BT. The low frequency band passes through'low pass lter E', transmission loss equalizer 111, delay path 110, low pass filter E, and amplifier A-118 which feeds through BT to line 1. The mid-fre'- quency band passes through band pass lter lil-,'delay path 112, band pass filter F, and amplifier A118 which feeds through B T to line 1. r

The east terminal of the system (not shown) is like the west terminal except that the delays introduced in the three frequency based on delaying the low frequency band by time t on transmlssion.

Condition I Condition II Delay Filters West set East set West set East set t I AA' (send) low `mid low low DD' (rec. high high mid high t-l-T BB (sen und low mid high EE' rec.) low mid high mid t+2T CC send) high high high mid FF (rec.)E mid low low low shifts which produce secrecy remains unchanged.

This differs from the case in which the time lags are applied in reverse order to the frequency bands in which case ,such reception doubles the time lag dierences.

The delay t is determined by the degree of clipping permissible. The delay t .prevents clipping of the low frequency ba'nd of energy transmitted through lilters A and A'. The energy passing through filters B and B and through filters C and C has additional tlme delay protection, against clipping, oftimes T and 2T. If switching is not to be combined with this circuit the time t may be made zero, i. e., the delay path designed to give the de- Y lay of time t can be omitted.

The transmission loss equalizers 107 and 111 may be made to have such loss that all of the frequency bands suffer equal transmission loss 1n .passing from line 1 to line 3 and also in passing from line 6 to line`1. -The gains of amplifiers .Ae-108 and A-118 may be adjusted to overcome those losses and give zero overall transmission losses, or desired gains, from line 1 to line 3 and from line 6 to line 1. v

By,closing switches 120 and 121 to connect with bridge transformer BT, the terminal shown in Fig. 4 maybe operated as a terminal of a two-wirev type'of system which maybe generally like that of which Fig. 3 shows onev terminal.

Where desired the delay :paths in ai? of the systems described herein may be o the mechanical or acoustic types, instead of elec' trical delaycircuits. YExamples'. of electrical delay paths that may -be employed in these systems, and of mechanical and acoustic delay paths that. may be employed systemssuch as those of Figs. 1, 2 and 3 which do not require two-way delay paths, .are disclosedv in Mathes Patent 1,696,315,1December 25,

1928. Examples of two-way electrical, me-

' and such an acoustic delay path is shown in netwon N, the

chanical and acoustic 'delay paths that have zero transmission loss (or a gain if desired) and are suitable for use in the delay paths in Fig. 4, as well as for use as the delay paths in Figs. 1, 2 and 3, are disclosed in my co'- pending application, Serial No. 351,417, tiled' of even date herewith, and an example of such an electrical delay circuit is disclosed in Clark Patent 1,672,057, June 5, 1928,

Fig. 5 described below and is claimed in m application just mentioned. Where suc lossless paths are used as the delay paths in Fig. 4, the amplifiers and transmission loss equalizers shown in that figure can be omitted from the system.

The delay circuit of Clark Patent 1,672,057 mentioned above, involves the addition of a 21-type telephone repeater to the system of Nyquist Patent 1,607,687, November 23, 1926 for getting twice the delay out of the particular delay circuit by a reectionprocess. Fig. 5 shows a similar system using an acoustic delay circuit of the general type disclosed in my Patent 1,696,315, mentioned above, instead of an electrical delay circuit. The delay path is shown connected between sections 150 and 151 of a two-way two-wire circuit or line.

l Speech energy comes from line section 150 into the h brid coil or bridge transformer circuit B and divides between two loud speaker elements 152 and 153, one coupled to an acoustic delay path 15,4 and the other coupled 'to an -acoustic resistance 155. This acoustic resistance, which is to' imitate or simulate the impedance of an infinite length of the acoustic delay path, might consist .of al substantially infinite pipe, or a pipe decreasing` exponentially in dlameter, or a composite group of open and 'short-circuited pipes as disclosed inthe copending application of W. P. Mason, Serial No. 221,261,`iled September 22, 1927, which issued as Patent No. 1,7 95,874, March 10, 1931. At the far end of the acoustic delay path .the speech energy is reconverted to the electrical form by another loud speaker elementl156 after which it is amplified by a 21-type telephone repeater 'l-157. Half of the amplified 1x5 energy sent on and lost in the balancing other .half returning via the l acoustic delay path 154 to the hy rid coil BT where half of it goes to the two-wire line section 151 and'half is fed back to the two- 120 wireline section 150.- The process for transmission from` line section 151 to line section 150 is exactly similar. This system not only has the property of a 21-type repeater cir-v cuit that energy -is talking as well as on to the subscriber listening, but furthermore has the property that a certain amount of direct transmissidn will pass between line" sections 150 and 151 through vthe hybrid- BT, its magnitude 190 y.

fed back to the subscriber `depending upon the degree lof balance obtamable, and .travel on down the circuit aheadjof the delayed transmission. The system shown has'been devised with a view to i 5 keeping this unbalance component as small as possible compared to the delayed main component. Ifdesired, an amplitude equalizer 158, which compensates for non-uniformv frequency-response characteristics of the loud speaker elements 152 and 156 and the delay path 154, can be included in lcircuit with amplifier A-157, as shown.

What is claimed is:

1. A,message path for altering the intelligibility of waves transmitted therethrough, and means for transmitting subdivided portions of message waves in opposite directions throughout the length of said path to increase the intelligibility of the messages in one direction and decrease the intelligibility of the messages inthe other direction, the portions of said path in which said increase and said decrease are effected being at least partly common.

2. A message distorting path for altering the time relation between waves trans- 'mitted therethrough to alter their intelligibility, and means for transmitting messages one direction an'd decrease' the intelligibility of the messages inthe other direction, the

portions of said path in which said increase location, for altering the time relation of waves transmitted in the opposite directionv 1 therethrough, of means for transmitting.

waves representing a message in one direction through said path, and means for transmitting waves representing a message in the opposite direction through said path, said path comprising channels having their trans.- mission frequency ranges mutually exclusive.

4. A two-way signaling' system comprising two intercommunicating stations, each. station com rising a message-path for alteringl the intelllgibility of waves transmitted theresage waves in opposite directions throughout the length of said p ath to increasethe intel- 'my name this 27th da of March, 1929.

ligibilityI of the'messages in one direction and decrease the `intelligbility. ofthe messages inthe other direction, the portions of 05 said path in which'said increase and said dein opposite directions therethrough to in= crease the intelligibility of .the messages in through, and means for transmitting mescrease are effected being at least partly common, `said path comprising channels having their transmission frequency ranges mutually exclusive, and said means at said two stations decreasing the intelligibility of messages in different manners for .the two directions, respectively. Q

5. A message path for altermg the intelligibility of waves transmitted therethrough, land means for transmitting message waves in opposite directions therethrough to increase the intelligibility of the messages in one direction and decrease the .intelligibility Iof the messages in the other direction, the portions of said path in which said increase and said decrease are effected being partly common and partly separate.

6. `A two-way signaling system comprising means responsive to signals transmitted in one direction in the system for preventing transmission of those` signals in the opposite direction in thel system, means for4 producing relative time delay of frequency bands of v energyof sai'd signals, and a delay pathcommon to sai-dL-two means.

7.. A` signaling system comprising means responsive to signalstransmitted'irr one directionin the system for rendering the system operative to transmit said4 signals in that direction, through a portion of the. system, means fr relatively delaying frequency bands of energy of said signals in their transmission, and la delay. path 4common to said two means. L

8. In combination, a message path for altering. the intelligibility of waves transmitted therethrough, and means for transmitting message waves in opposite directions throughout the length .of said path to increase the intelligibilty of the messages in one direction and decrease the intelligibility of the messages in the-other direction, the portions of said path in which saidincrease and said decrease, are effected' being at least partly common, and means responsive to message waves transmitted through said path for preventing simultaneoustransmission in opposite directions therethrough.

signal transmitting means for distorting signals transmitted in one direction over the system to render them unintelligible, signal transmitting means for differently distorting signals transmitted in the opposite direction over the system to render them-unintelligible, and means responsive to signals trans- 9. A two-way signaling system comprising '4 l mitted-in said system for preventing .simulytaneou's transmission in opposite directions through either. of said two signal transmitting and distorting means. j

In witness whereof, I hereunto `subscribev 

